Abstract:
The first geologic map of Cape Lamb, SW Vega Island, is presented along with their description and interpretation. The main outcrops correspond to Late Cretaceous sedimentary rocks and neogene volcanic rocks.

The former, mainly sandstones and mudstones deposited in shelf environments during Maastrichtian times, are sudbivided into three informal members (K1, K2 and K3) and assigned to the ... Marambio Group. The identification and descrition of nine sedimentary facies allowed an environmental restoration of the area and two complete transgressive-regressive cycles (K1+K2 and K3) are identified. The erosive surface bounding these cycles is covered by a transgressive lag deposit and can be interpreted either as a wave ravinement surface (transgressive erosional surface) or as a coplanar (sequence boundary+flooding surface) surface. The Late Cretaceous rocks contain a very abundant and diverse invertebrate marine fauna and have yielded a unique assemblange of terrestrial and marine vertebrates (plesiosaurs, mosasaurs, dinosaurs and birds). A list of the vertebrates and invertebrates recognized up to now with their stratigraphic distribution is also presented. These rocks correlate with those cropping out at the northern part of the Cerro Nevado (Snow Hill) and southern part of the Marambio (Seymour) islands towards the basin center. Based on the joint occurrence of Gunnarites antarcticus and Jacobites crofti, K2 is correlated with the Karlsen Cliff and Haslum Crag members of the Snow Hill Island Formation, while palynomorphs and the amonnoid Maorites densicostatus allow to link K3 with medium levels (KLB6-7) of the Lopez de Bertodano Formation. Therefore, the lag conglomerate at the base of K3 could represent a condensed deposit equivalent to part of the Haslum Crag Member of the Snow Hill Island Formation and the lower, muddy, section (KLB1-4) of the Lopez de Bertodano Formation at the Cerro Nevado-Marambio section.

The latter, assigned to the James Ross Volcanic Group, are composed of glaciomarine diamictites (Ta), intercalated within volcanic and volcaniclastic depostis and basaltic dikes (Tb). The history of the GVJRI at Cape Lamb is the result of three eruptive events. The large thickness of pyroclastic deposits indicates a clear dominance of the explosive facies. The presence of sedimentary structures and compositional studies allow to relate these deposits to subacueous and subglaciar processes. According to textural and compositioanl characteristics, the tuffs are product of base surge whereas the breccias may result from autobreccia of basalt flows entering the glaciomarine environment. Subareal facies are minor participants represented by basalt flows and subvolcanic bodies with fine columnar jointing. The geomorphology of the area is modelled mainly by glacial, marine and fluvial processes along with pervasive slumping.

This paper presents and describes the distribution and composition of the different units, interpreting their origin and correlation with other parts of the James Ross Basin.